I have also heard that it began referring to the length of fabric used for a Scottish great kilt, (not the modern kilt) . With the great kilt, there is a great deal of fabric that is tossed over the shoulder and can be used like either a shawl, blanket, or just tossed over the shoulder and secured with a broach. The bottom edge of a kilt isn't hemmed, it is called the salvage (not sure of the spelling) very edge of the bolt of fabric. Cheaper kilt makers would not use the entire 9 yards of wool in the bolt of fabric, and would try to get away with skinnier pleats and use less fabric for the "fly" part of the kilt that goes over the shoulder.
I have only worn modern kilts, so I have no idea if any of that is true, or just Scottish BS. I do know that one of my kilts is made with 22 ounce wool, that is as heavy as the wool used on a man's heavy winter coat. It weighs about 5-6 pounds and is REALLY warm. The pleats over the back side are 6-7 layers of wool thick (about .5-.75" thick of wool). Most modern kilts are made with 12-16 ounce wool...the really heavy one I have is a reproduction of a WWI.WWII military black watch kilt. In any case, that kilt is warm enough that I was comfortable in 8" of snow and 25 degrees farenheit. But it also requires proper hat, knee hose of similar weight, and upper body garments.
The ammo belt reference makes more sense to me than the kilt reference...